And Paladins have zero interaction with Shield or Sword Oath. That's because Shield Oath, Sword Oath, and Grit are all not the core mechanics of Paladin and Dark Knight. Their core mechanics are both MP management and ability management, with Dark Knight having a higher emphasis on the MP management through Dark Arts.
Meanwhile, Warrior's core mechanic is their stances, Defiance and Deliverance play an integral role in the Warrior's kit. Even before Heavensward, Warriors relied on Defiance to flesh out and complete their kit. Dark Knights have little interaction with Grit while Paladins have no interaction with Shield Oath and Sword Oath beyond their defensive, offensive, and enmity boosts.
Now, if you want to talk about possible balance issues, I think it wouldn't be overly unbalanced if Dark Knights had half restorative power on Souleater without Dark Arts while outside of Grit, and only quarter power with Dark Arts while outside of Grit. Then you just write the Grit effect to "Dark Arts Effect absorbs 100% of damage dealt as HP." This would put Dark Arts as a closer comparison to Storm's Path and Inner Beast bundled up into one neat little package.
And yet, it's fine for the other tank without a shield to have twice the duration of Bloodbath, Berserk, a stance that boosts their damage by another 5%, and again, a spammable combo that restores half the damage dealt as HP with only 10 less potency that also reduces the target's damage output by 10%. And their other HP restoration attack ignores their tank damage reduction, reduces their damage incoming by 20%, and heals them, again, for 100% of the damage dealt.
Edit: Further thought about this, mostly by beating up on a lv60 dummy. Giving Dark Knights 50% absorption outside of Grit and 25% for Souleater would actually make for an interesting dynamic. My Warrior has a weapon 10 item levels below my Dark Knight (Axe of the Sephirot vs Hyperconductive Nothung), and the HP restoration with my proposed numbers are about the same, with the Dark Arts version of Souleater actually being considerably weaker than without, which is a fair trade off for the amount of power the Dark Knight gains from Dark Arts.